1. Field of the Invention
The present invention relates to a process for forming a semiconductor film, particularly a carbon-containing silicon film and a semiconductor device comprising the semiconductor film, and more particularly to a process for forming a semiconductor film having a wide forbidden band gap, such as non-monocrystalline silicon carbide film and a semiconductor device comprising the semiconductor film.
2. Related Background Art
Polysilicon and silicon-based, non-monocrystalline films such as amorphous silicon film are widely applied to photosensitive members, thin film transistors, sensors, photovoltaic elements, etc. A non-monocrystalline silicon carbide film (which will be hereinafter referred to as a-Si.sub.1-x C.sub.x film) has been used as a material for the electro-charging layer of the photosensitive member and as a material on the light incident side of a solar cell.
Conventionally, a-Si.sub.1-x C.sub.x films are deposited by decomposition reaction of a Si-based gas such as a monosilane gas, a disilane gas, a fluorosilane gas, etc. and a hydrocarbon of linear structure such as methane, ethane, propane, butane, ethylene, acetylene, etc. as raw materials under light irradiation or application of high frequency (JP-A-57-103311; JP-A-59-172275; No. 39 meeting of society of applied physics (1992), Lecture Summary, page 775, 31p-ZT-10). It is also proposed to conduct the deposition by decomposition reaction of an organic silane having a hydrocarbon group such as an alkyl group or an aryl group in the molecule as a raw material (JP-A-62-113482; JP-A-2-3912). It is also proposed to deposit an a-Si.sub.1-x C.sub.x film from a carbon raw material gas comprising acetylene as the main component and linear hydrocarbons such as C.sub.2 H.sub.6, C.sub.3 H.sub.8, C.sub.4 H.sub.10, C.sub.5 H.sub.12, C.sub.2 H.sub.4, C.sub.3 H.sub.6, C.sub.4 H.sub.8 and C.sub.5 H.sub.10 or their isomers in a mixing ratio of the former to the latter of 0.1 to 10 by volume (JP-A-63-136514).
However, in the formation of an a-Si.sub.1-x C.sub.x film using linear hydrocarbons as a carbon raw material gas or an organic silane, the bonding forces of C--H and C--C are stronger than those of Si--H and Si--Si and undergo disassociation with difficulty. That is, the decomposition efficiency of hydrocarbon is so low that with the increasing number of carbon atoms in the hydrocarbons the hydrocarbon gas must be supplied in excess to the silane-based gas. It is also difficult to control the film growth conditions.
Furthermore, a probability of taking the bonds of C--H and C--C as such into the resulting film is high. The C--H bond is hard to break and carbon atoms are often taken into the film together with a plurality of hydrogen atoms in the form of C--Hn, where n=1, 2 or 3. In the non-monocrystalline hydrogenated silicon-based materials, excess hydrogen is generally said to be an electroconduction-inhibiting factor. Particularly in an a-Si.sub.1-x C.sub.x film having a high carbon content an excess hydrogen tendency is pronounced, It has been presumed to be difficult to obtain an a-Si.sub.1-x C.sub.x film having a wide band gap and very good electrical characteristics.
In the deposition of an a-Si.sub.1-x C.sub.x film from a mixture of acetylene as the main carbon raw material with other hydrocarbons, many more unsaturated carbon bonds are taken into the resulting film, thereby preventing formation of an ideal network of Si--C bonds. That is, voids are formed around the unsaturated bonds. The voids inhibit electroconduction and thus it is also hard to obtain an a-Si.sub.1-x C.sub.x film having very good electrical characteristics using this method.